A critical race condition vulnerability (CVE-2025-55680) in Microsoft Windows Cloud Minifilter (cldflt.sys) allowed attackers to exploit a time-of-check time-of-use (TOCTOU) weakness during placeholder file creation in cloud synchronization services like OneDrive. By manipulating filenames in memory between validation and file creation, attackers could bypass security checks and write arbitrary files—including malicious DLLs—to restricted system directories (e.g., *C:\Windows\System32*). This enabled privilege escalation to **SYSTEM-level access**, permitting arbitrary code execution.The flaw stemmed from inadequate filename validation in the *HsmpOpCreatePlaceholders()* function, a regression linked to a prior patch (CVE-2020-17136). Exploitation required only basic user privileges, posing severe risks to multi-user environments. Microsoft addressed the issue in the **October 2025 security updates**, but unpatched systems remained vulnerable to attacks leveraging DLL side-loading techniques. Organizations using cloud sync services with configured sync root directories were at heightened risk, as these were prerequisites for successful exploitation. The vulnerability carried a **CVSS 3.1 score of 7.8 (High)** and threatened system integrity, confidentiality, and availability through unauthorized privilege escalation.
Source: https://cyberpress.org/windows-cloud-minifilter-flaw/
TPRM report: https://www.rankiteo.com/company/microsoft
"id": "mic3832638102125",
"linkid": "microsoft",
"type": "Vulnerability",
"date": "6/2020",
"severity": "60",
"impact": "",
"explanation": "Attack with significant impact with internal employee data leaks:"{'affected_entities': [{'customers_affected': 'Users of Windows systems with '
'cloud synchronization services '
'(e.g., OneDrive)',
'industry': ['Technology',
'Software',
'Cloud Services'],
'location': 'Redmond, Washington, USA',
'name': 'Microsoft',
'size': 'Large (Enterprise)',
'type': ['Corporation', 'Software Vendor']}],
'attack_vector': ['Local',
'Time-of-Check Time-of-Use (TOCTOU)',
'Multi-threaded Exploitation'],
'customer_advisories': ['Users of Windows cloud synchronization services '
'(e.g., OneDrive) should apply the October 2025 '
'updates to mitigate the risk of privilege '
'escalation.'],
'date_detected': '2024-03',
'date_resolved': '2025-10',
'description': 'A critical security flaw in Microsoft Windows Cloud '
'Minifilter (cldflt.sys) was fixed, addressing a dangerous '
'race condition (CVE-2025-55680) that enabled attackers to '
'gain elevated system privileges (SYSTEM-level) and write '
'files to any location on affected systems. The vulnerability, '
'discovered by Exodus Intelligence in March 2024, was patched '
"in Microsoft's October 2025 security updates. It arises from "
'inadequate filename validation during placeholder file '
'creation in cloud synchronization services (e.g., OneDrive), '
'allowing attackers to exploit a time-of-check time-of-use '
'(TOCTOU) weakness via multi-threaded attacks. This could lead '
'to arbitrary DLL placement in restricted directories (e.g., '
'C:\\Windows\\System32) and privilege escalation through DLL '
'side-loading. The flaw impacts systems running cloud sync '
'services with configured sync root directories and relates to '
'a previously patched issue (CVE-2020-17136).',
'impact': {'brand_reputation_impact': ['Potential reputational damage for '
'Microsoft due to critical '
'vulnerability in core cloud sync '
'functionality'],
'operational_impact': ['Potential SYSTEM-level privilege '
'escalation',
'Arbitrary file creation in restricted '
'directories (e.g., C:\\Windows\\System32)',
'DLL side-loading attacks'],
'systems_affected': ['Windows systems running cloud '
'synchronization services (e.g., OneDrive)',
'Systems with configured sync root '
'directories']},
'investigation_status': 'Resolved (Patch Released)',
'lessons_learned': ['Race conditions in validation logic can reintroduce '
'vulnerabilities even after prior patches (e.g., '
'CVE-2020-17136).',
'Cloud synchronization services introduce attack surfaces '
'that require rigorous input validation, especially for '
'file operations.',
'Time-of-check time-of-use (TOCTOU) vulnerabilities can '
'be exploited with multi-threaded techniques to bypass '
'security controls.',
'Privilege escalation via DLL side-loading remains a '
'persistent risk when attackers can write to system '
'directories.'],
'post_incident_analysis': {'corrective_actions': ['Microsoft released a patch '
'in October 2025 to address '
'the race condition in '
'filename validation.',
'Enhanced input validation '
'for placeholder file '
'operations in cloud sync '
'services.',
'Security hardening of the '
'CfCreatePlaceholders() API '
'and related I/O control '
'codes.'],
'root_causes': ['Inadequate filename validation in '
'the HsmpOpCreatePlaceholders() '
'function during placeholder file '
'creation.',
'Race condition (TOCTOU) between '
'filename validation and actual '
'file creation in the Windows '
'Cloud Minifilter driver '
'(cldflt.sys).',
'Multi-threaded attack surface '
'enabled by the '
'CfCreatePlaceholders() API and '
'I/O control code 0x903BC.',
'Incomplete fix for a prior '
'vulnerability (CVE-2020-17136) '
'reintroduced the race '
'condition.']},
'recommendations': ["Apply Microsoft's October 2025 security updates "
'immediately to all Windows systems.',
'Prioritize patching for systems with cloud '
'synchronization services (e.g., OneDrive) and configured '
'sync root directories.',
'Monitor for suspicious file creation activities in '
'system directories (e.g., C:\\Windows\\System32).',
'Implement least-privilege principles to limit the impact '
'of potential privilege escalation attacks.',
'Conduct security reviews of cloud sync integrations to '
'identify similar validation gaps.',
'Educate system administrators on the risks of TOCTOU '
'vulnerabilities in file operations.'],
'references': [{'source': 'Exodus Intelligence (Vulnerability Discovery)'},
{'source': 'Microsoft Security Update Guide (CVE-2025-55680)'},
{'source': 'Microsoft Security Update (October 2025)'}],
'response': {'containment_measures': ['October 2025 security updates (patch '
'release)'],
'remediation_measures': ['Apply Microsoft security updates '
'(October 2025)',
'Prioritize patching systems with cloud '
'sync root directories'],
'third_party_assistance': ['Exodus Intelligence (vulnerability '
'discovery)']},
'stakeholder_advisories': ['Microsoft recommends immediate patching for all '
'affected systems.'],
'title': 'Critical Race Condition Vulnerability in Microsoft Windows Cloud '
'Minifilter (CVE-2025-55680)',
'type': ['Vulnerability', 'Privilege Escalation', 'Race Condition'],
'vulnerability_exploited': {'affected_component': ['Microsoft Windows Cloud '
'Minifilter (cldflt.sys)',
'HsmpOpCreatePlaceholders() '
'function',
'CfCreatePlaceholders() '
'API'],
'cve_id': 'CVE-2025-55680',
'cvss_score': {'score': 7.8,
'severity': 'High',
'version': '3.1'},
'related_vulnerabilities': ['CVE-2020-17136'],
'vulnerability_type': ['Race Condition',
'Improper Input '
'Validation']}}